The present invention relates to gas burners such as the type found in gas furnaces, and is more particularly concerned with means for electronically igniting the burner and for detecting or proving the existence of flame after ignition.
A number of electric igniter systems have been proposed for use with gas burners, including igniters that employ a high voltage spark, and igniters that involve a hot surface. In a mobile environment, in which the power for the furnace or heater is derived from a 12 volt DC or a 24 volt DC source, it has been common to employ a spark igniter, as heated surface type igniters have a high failure rate. The spark igniter requires some source of AC or pulsating voltage, and an inverter can be used to generate a wave which is then fed to an ignition transformer. Because of the relatively low voltage available in the mobile environment (i.e., 12 or 24 VDC), the turns ratio of the ignition transformer needs to be quite high. This means that the cost of the transformer is quite high, and also that the transformer can experience inter-turn arcing if fine wire is used in the secondary winding.
In any gas furnace it is mandatory to detect a successful ignition as a safety measure. If gas is permitted to flow to an unlit burner, explosive vapors can fill the dwelling and create a hazardous situation. Accordingly, a flame detection or flame proving means needs to be employed at the gas burner. One simple means for doing this is with a flame rectification probe. This technique is based on the fact that an active flame acts as a plasma diode. A unidirectional current can flow from a probe within the flame to the metal casing of the burner, i.e., the firebox. The flame itself thus acts like a resistance and diode connected in series. By applying an alternating current to the rectification probe, it is possible to detect the presence of flame. Rectification flame proving requires a source of alternating current, but in a mobile environment, where the power comes from 12 or 24 VDC, an inverter or other AC source has to be included in the burner control circuitry. This increases the cost of the circuitry. Moreover, the additional circuit elements increase the risk of failure.
Accordingly, a low cost ignition circuit and a flame detection circuit that would be suitable in a DC control system have been sought without success. A DC furnace control circuit that combines a burner igniter and a flame rectification probe has also been unavailable, without use of an on-board transformer.